Remove no-longer-needed allow(dead_code) from the standard library

`repr(transparent)` now silences the lint.

library/alloc/src/boxed/thin.rs

@@ -171,7 +171,6 @@ struct WithHeader<H>(NonNull<u8>, PhantomData<H>);
 /// An opaque representation of `WithHeader<H>` to avoid the
 /// projection invariance of `<T as Pointee>::Metadata`.
 #[repr(transparent)]
-#[allow(dead_code)] // Field only used through `WithHeader` type above.
 struct WithOpaqueHeader(NonNull<u8>);
 
 impl WithOpaqueHeader {